In a CDMA (Code Division Multiple Access) mobile communication system, transmission power control is an important technology from the standpoint of expanding system capacity. One transmission power control method is the outer loop transmission power control method. With the outer loop transmission power control method, mobile station apparatuses and a base station apparatus vary the reference SIR (Signal to Interference Ratio) according to quality that varies according to the propagation environment in order to maintain the receiving line quality (abbreviated to “quality” below), such as the FER (Frame Error Rate) for example, at a constant level, and transmission power control is performed in accordance with the result of comparing that variable reference SIR with the reception SIR. The reason for varying the reference SIR so that mobile station apparatuses and the base station apparatus maintain quality at a constant level is that, since transmission of excessive quality causes a slight increase in interference with other stations, it is necessary to transmit at the minimum transmission power that satisfies the required quality.
A conventional radio communication apparatus that performs outer loop transmission power control will be described below. FIG. 1 is a principal block diagram showing the outline configuration of the reception system of a conventional radio communication apparatus. In FIG. 1, a demodulation section 11 performs predetermined demodulation processing on a received signal. A decoding section 12 decodes data after demodulation.
A CRC judgment section 13 performs a CRC (Cyclic Redundancy Check) on decoded data, judges whether or not there is an error in the data, and outputs data after judgment in decoding units. If an error is detected (if CRC=NG), an increment/decrement value calculation section 14 generates a reference SIR increment value S1, expressed by equation (1) below. If an error is not detected (if CRC=OK), the increment/decrement value calculation section 14 generates a reference SIR decrement value S2, expressed by equation (2) below.                     S1        =                  0.5          ⁡                      [            dB            ]                                              (        1        )                                                                    S2              =                            ⁢                                                -                  S1                                *                FER_TARGET                ⁢                                  /                                                                                                                      ⁢                                                (                                      1                    -                    FER_TARGET                                    )                                ⁡                                  [                  dB                  ]                                                                                                        FER_TARGET              =                            ⁢                              10                                  -                  3                                            ⁢                              (                                  in                  ⁢                                                                           ⁢                  case                  ⁢                                                                           ⁢                  of                  ⁢                                                                           ⁢                  voice                  ⁢                                                                           ⁢                  communication                                )                                                                                        =                            ⁢                              10                                  -                  4                                            ⁢                              (                                  in                  ⁢                                                                           ⁢                  case                  ⁢                                                                           ⁢                  of                  ⁢                                                                           ⁢                  data                  ⁢                                                                           ⁢                  communication                                )                                                                        (        2        )            
Equations (1) and (2) above are computational equations generally used as reference SIR increment/decrement value computational equations in outer loop transmission power control. FER_TARGET indicates the Frame Error Rate (quality) that it is wished to maintain at a constant level for the radio communication apparatus.
A reference SIR update section 15 updates the reference SIR value by adding a reference SIR increment/decrement value determined by above equation (1) or (2) to the current reference SIR value. The update cycle is generally the CRC judgment cycle of 10 ms. The updated reference SIR value is output to a comparison section 17.
The comparison section 17 compares the received signal SIR value measured by an SIR measurement section 16 with the updated reference SIR value, and outputs the result of the comparison to a transmission power control bit generation section 18. If the measured SIR value is larger than the reference SIR value, the transmission power control bit generation section 18 generates a transmission power control bit directing the communicating party to decrease transmission power. If the measured SIR value is equal to or smaller than the reference SIR value, the transmission power control bit generation section 18 generates a transmission power control bit directing the communicating party to increase transmission power.
The transmission power control bit generation section 18 then outputs the generated transmission power control bit to the transmission system of the radio communication apparatus. In the transmission system, the transmission power control bit is mapped onto the transmit signal. The communicating party adjusts the transmission power according to the received transmission power control bit. Conventionally, outer loop transmission power control is performed in this way between radio communication apparatuses.
However, there is a problem with the conventional radio communication apparatus and outer loop transmission power control method, as follows.
FIG. 2 is a drawing showing variations in the reference SIR when outer loop transmission power control is performed using a conventional radio communication apparatus. FIG. 2 shows the case of an 80 ms decoding unit and 10 ms frame: that is, the case where eight CRC judgments (1) to (8) are performed at 10 ms intervals within 80 ms. Time is shown on the horizontal axis and reference SIR values on the vertical axis.
If an error is detected in CRC judgments (3) to (5) (if CRC=NG), a reference SIR increment value S1 is calculated with equation (1) above. That is to say, in CRC judgments (3) to (5) the reference SIR values are each incremented by 0.5 [dB].
If an error is not detected in CRC judgments (6) to (8) (if CRC=OK), a reference SIR decrement value S2 is calculated with equation (2) above as shown below.S2=−0.5*10−4/(1−10−4)≈−{fraction (0.5/10000)} [dB]  (3)
where FER_TARGET=10−4 (in case of data communication)
That is to say, in CRC judgments (6) to (8) the reference SIR values are each decremented by approximately {fraction (0.5/10000)} [dB].
As can be seen from the result of equation (3) above, the reference SIR values are decremented on an extremely gentle gradient. Therefore, with a conventional radio communication apparatus, once the receive data quality becomes excessive, an extremely long time is required for the reference SIR value to be set to the optimum reference SIR value. That is, with a conventional radio communication apparatus, the time during which data is transmitted at excessive transmission power is extremely long, and therefore interference with other stations increases and system capacity decreases.